Recently, research into 3D image reconstruction application algorithms using ultrahigh frequency/millimeter wave/terahertz frequency, and so on, has been rapidly conducted. Due to features of electromagnetic waves, an electromagnetic wave scattered from a target object has a variety of geometric/physical features of the target object. However, due to the scattering feature of the electromagnetic wave, amplitude or phase of a received scattered electromagnetic wave is not direct 3D image of the target object.
An image reconstruction method refers to a method that reconfigures a received scattered electromagnetic wave using an algorithm and three-dimensionally finds geometric/physical features of a target object. A 3D image reconstruction belongs to a very mathematically difficult field among electromagnetic technology fields. In order to perform a 3D image reconstruction, an integral equation including scattering needs to be solved based on a numerical analysis.
Meanwhile, since the numerical analysis of the integral equation is very time-consuming in computation, a far-field approximation is forcibly applied such that the numerical analysis is applicable to a Fast Fourier Transform (FFT) algorithm having a fast computation time. In this case, in order for exact application of the FFT algorithm, a measurement distance between an electromagnetic wave transmitting/receiving unit and a target object needs to be longer than a wavelength of an electromagnetic wave. If the measurement distance is short, image reconstruction accuracy is lowered due to a far-field approximation error.
Measurement values may not be obtained periodically, depending on a method of measuring a scattered electromagnetic wave. Therefore, an interpolation is additionally applied to obtain periodic measurement values. This is because a high-speed FFT is applicable only when the measurement values are periodic. However, due to the application of the interpolation, a converted value based on the measurement value is used as an input of the FFT. Therefore, an image reconstruction resolution of a 3D target object may be lowered.
In order to perform a 3D image reconstruction at a high speed and high resolution with respect to any measurement distance, it is necessary to solve an integral equation for inverse scattering exactly and quickly, without far-field approximation. Therefore, there is a strong need for high resolution image reconstruction technologies based on an analytic integral equation solution, and high speed image reconstruction technologies using no FFT.